Extreme ultra violet (hereinafter, EUV) lithography is an exposure technique utilizing EUV light and has a potential in application in the semiconductor industry which has recently pursued the manufacturing of semiconductor devices with finer designs. Here, the term “EUV light” indicates light having wavelengths in the soft X-ray region or the vacuum UV region, specifically, wavelengths of about 0.2 to 100 nm. For example, Patent Literature 1 proposes a reflective mask for exposure as a mask used in the EUV lithography.
In the reflective mask described in Patent Literature 1, a multilayer reflective film capable of reflecting exposure light is disposed on a substrate, and a pattern of an absorber film capable of absorbing the exposure light is disposed on the multilayer reflective film. The light incident on the reflective mask mounted on an exposure apparatus (a pattern transfer apparatus) is partly absorbed by the absorber film and is partly reflected by the multilayer reflective film exposed from the absorber film. The light reflected by the portions exposed from the absorber film is transferred onto a semiconductor substrate via a reflective optical system.
Patent Literature 2 describes a mask blank defect inspection method including a step of applying EUV light from a light source to a prescribed inspection region of a mask blank. Further, Patent Literature 2 describes that recesses with a narrow width are formed beforehand by a method such as a focused ion beam (FIB) process in portions of the surface of a mask substrate (a super smooth substrate) for constituting the mask blank, and a multilayer film is deposited over the recesses for forming fiducial marks.
Patent Literature 3 describes a reflective mask blank substrate for EUV lithography characterized in that at least three marks satisfying prescribed conditions are disposed on a film-production surface of the substrate. The prescribed conditions are (1) that the size of the marks is 30 to 100 nm in terms of circular equivalent diameter and (2) that three of the marks on the film-production surface are not on the same virtual straight line. Further, Patent Literature 3 describes that the prescribed marks are formed by a lithography process.